1. Field of the Invention
The present invention relates to flexible-tube devices of the "push-pull" cable type for the transmission of tractive forces and/or compressive forces. A device of this type consists of a flexible tubular casing or sheath which has a circular cross-section and is formed of either metal or plastic material. A flat and flexible central member usually formed of high-strength stainless steel and designated as a core strip is placed in the axis of the tubular sheath. The function of the core strip is to transmit forces as it moves axially with respect to the stationary sheath between two rows of ball-bearings which are guided within ball-races cut in both faces of the core strip.
The balls which roll in contact with the core strip are held at a fixed distance from each other by means of a ball-bearing retainer cage usually consisting of a flat strip provided with uniformly spaced holes for receiving the balls.
2. Description of the Prior Art
In a known form of construction (as described in French patent No. 1,124,613 filed on Apr. 8th 1955 in the name of the same inventor), the two rows of ball-bearings are adapted to move on the one hand in ball-races cut in both faces of the core strip and, on the other hand, in the ball-races formed in two stationary oppositely-facing flexible outer strips. These flat race strips are usually formed of stainless steel and applied against the circular internal face of the tubular sheath.
In known transmission devices of this type, the two ends of both outer race strips are engaged and locked with terminal collars, said collars being in turn secured to the frame of the machine on which the transmission device is mounted whilst the two ends of the core strip are rigidly fixed respectively to two sliding rods mounted in the axis of said terminal collars and coupled respectively to the actuating elements or elements to be actuated.
The transmission assembly which is formed of flat strips can assume a curved or sinuous configuration by virtue of the fact that the retaining means provided for the race strips and the core strip are capable of self-orientation by rotating about the axis of the terminal collars with a view to ensuring that the plane containing the two rows of ball-bearings is located in the plane of the curve of the flexible sheath as described in French patent No. 1,212,730 filed on Mar. 25th 1958 in the name of the same inventor.
In another known design such as the assembly described in French patent No. 1,410,294 filed on July 27th 1964 in the name of the same inventor, the two rows of balls retained by the cage strips roll on the one hand in raceways cut in both faces of the core strip and, on the other hand, directly against the internal face of the flexible tubular sheath. Said sheath is so designed as to have on the one hand an internal surface having the function of a raceway which is suited for the balls and on the other hand a sufficient degree of mechanical strength to withstand the various stresses to which the sheath may be subjected, namely compressive stresses, tensile stresses and bending stresses.
In consequence, ball-bearing transmission systems of the known push-pull cable type either comprise two outer ball-race strips or else are not provided with any outer race strip when the ball-bearings roll directly in the tubular sheath. Up to the present time, the structure of all known ball-bearing transmission systems has therefore been perfectly symmetrical with respect to a mid-plane which passes through the core strip. In point of fact, this symmetrical structure has always seemed both logical and indispensable for a flexible transmission system.
The two known solutions (with two outer ball-race strips or with no outer strip) are both attended by advantages as well as disadvantages from the point of view of bulk, cost price, forces to be transmitted, and so on.